EPR detection of heme and nonheme iron-containing protein nitrosylation by nitric oxide during rejection of rat heart allograft.

The paramagnetic molecule nitric oxide (NO), produced from L-arginine by a specific enzyme (NO synthase), has been shown to be involved in a surprising variety of mammalian cellular responses, including the regulation of T cell immunity to alloantigens in vitro. In cytotoxic activated macrophages, NO production results in a characteristic pattern of alteration of iron-containing enzyme function that is mimicked by exposure to NO. Electron paramagnetic resonance (EPR) studies have shown the formation of iron-nitrosyl species during macrophage activation and also during sepsis, indicating that alteration of iron-containing protein function may be the result of the well-documented tendency of NO to bind to metal ions. We have recently shown that the NO synthesis induced during alloantigenic activation of rat splenocytes inhibits lymphocyte proliferation and cytotoxic T-lymphocyte generation. This report demonstrates that iron-nitrosyl EPR signals similar to those observed in macrophages and during sepsis are present in the blood and in the grafted tissue of rats during the rejection of allogeneic (but not syngeneic) heart grafts. These signals are found in the blood and at the site of allograft rejection, but are not found in other tissues (such as spleen and lung), and are obliterated by administration of the immunosuppressant FK506. These results directly demonstrate the formation of iron-nitrosyl complexes during vascularized allograft rejection and suggest that consequent destruction of iron-containing protein function plays an important role in the rejection response.